1. Field of the Invention
The present invention relates to a dry etching method employed in such applications as production of semiconductor devices. More particularly, it relates to a dry etching method whereby a nitrogen based compound film can be substituted for a resist mask as an etching mask, thereby preventing promotion of carbon pollution and deterioration of selectivity for an underlying layer and inhibiting after-corrosion.
2. Description of the Related Art
The recent trend toward higher integration and performance of such semiconductor devices as VLSIs and ULSIs requires a dry etching method whereby correspondingly higher anisotropy, higher etchrate, higher selectivity, lower pollution, and less damage can be achieved with no compromise in these requirements.
Most typical of the conventional dry etching methods is to etch a target material layer via a resist mask patterned into a predetermined shape. In this method, etching the target material layer into an isotropic shape requires applying a high bias power thereto under low gas pressure to increase the mean free path of ions and the energy of incident ions. At this time, high-energy incident ions will sputter the resist mask, composed of organic materials, and form a carbonaceous decomposition product, which will, in turn, deposit on pattern sidewalls as a carbonaceous polymer, providing sidewall protection effects and contributing to isotropic etching.
In recent years, however, it has been revealed that carbon present in the etching system causes various problems.
One of the problems is a deterioration of the selectivity for silicon oxide (SiO.sub.2) in the presence of carbon. This phenomenon is reported in the Extended Abstract of the 36th Spring Meeting (1989) of the Japan Society for Applied Physics and Related Societies, Vol. 2, p. 572, 1p-L-7 and January's issue of Monthly Magazine "Semiconductor World" (published by Press Journal Inc.) (1990), p. 81-84. The cause of this phenomenon is that an SiO.sub.2 gate oxide film, which would not be etched in theory, is etched slightly when a gate electrode formed thereon is etched by using HBr gas. This cause is somewhat predictable from the descending order in binding energy of an Si--O bond (111 kcal/mole), Si--Br bond (88 kcal/mole), and Si--Si bond (54 kcal/mole). As a result, when an Si based material layer formed on an SiO.sub.2 based material layer is etched by using HBr gas, high selectivity can be expected for the underlying layer, but its actual value is in the order of 10 to 20.
According to the above mentioned sources, such low selectivity for the SiO.sub.2 based material layer can be attributed to carbon pollution. More specifically, any carbon adsorbing on the surface of the SiO.sub.2 based material layer will form a C--O bond (257 kcal/mole), which will override an Si--O bond with a smaller binding energy and form an Si--Br bond with an even smaller binding energy. To improve the selectivity for the SiO.sub.2 based material layer, the above-mentioned Extended Abstract proposes the measure of using HBr gas with higher purity, an etching chamber and pipes of pollution-free material, and an SiO.sub.2 mask as a substitute for a resist mask.
Another problem due to carbon is promotion of after-corrosion in etching an aluminum (Al) based material layer.
Generally, an Al based material layer is etched by using chlorine based gases, typically, BCl.sub.3 /Cl.sub.2 mixed gas. After completion of etching, an etching reaction product AlCl.sub.3, etching gas decomposition products, etc. will inevitably remain on the surface of the wafer carrying the Al based material layer. These residues will not only adsorb on the surface of the wafer but also occlude on the inside of the resist mask and carbonaceous polymer acting as a sidewall protection film. The resulting residual chlorine will absorb water in the air to form an electrolytic droplet, in which aluminum from the Al based material layer will be eluted to promote corrosion. The recent trend in etching the Al based material layer has been toward fabricating a barrier metal or anti-reflection film therewith and adding copper (Cu) thereto to prevent electro-migration. However, these additives will serve to promote more after-corrosion than prevent the after-corrosion.
The present inventor has proposed a dry etching method whereby an Al based material layer is masked with an SiO.sub.2 material layer containing spin on glass (SOG) and etched by using S.sub.2 Cl.sub.2, S.sub.2 Br.sub.2, and other sulfur halides while cooling a target wafer to temperatures below 0.degree. C. In this method, the SiO.sub.2 based material layer is substituted for a resist mask, reducing the quantity of residual chlorine. Further, the sulfur halides, when dissociated by electric discharges, will form free sulfur in a plasma for adsorption on the surface of a cooled wafer. The sulfur thus formed will replace the carbonaceous polymer in depositing on pattern sidewalls and providing sidewall protection effects, thus contributing to anisotropic etching.
Thus, various etching methods have been proposed wherein an SiO.sub.2 based material layer is used as an etching mask to avoid various problems due to carbon. However, there are some problems in putting an SiO.sub.2 mask to practical use in etching processes. First, in a gate electrode forming process using an SiO.sub.2 mask, since a formed gate electrode is eventually coated with an inter-layer insulation film, the SiO.sub.2 mask may be left unremoved without hindering production of a semiconductor device.
Nevertheless, it is preferred to remove the SiO.sub.2 mask in order to prevent an increase in the magnitude of wafer surface steps, which may deteriorate the accuracy of the step coverage photo-lithography for material layers formed in the subsequent processes or photo-lithography. However, any attempt to remove the SiO.sub.2 mask through etching will remove a thin gate oxide film as well, making this process impractical.
Meanwhile, in an Al based material layer etching process using an SOG mask, although after-corrosion is prevented effectively, another process is required for forming an SOG mask. It is also preferred to prevent such an increase in the number of necessary processes.